Cassel was the first Jewish honorary citizen of Berlin and a leading figure in several Jewish-German organisations.

His father was the rabbi of the Jewish community of Schwetz, he moved to Konitz (Chojnice) in 1850 and Flatow (Złotów) in 1854.

Representing the "Verband der deutschen Juden" Cassel protested against the Judenzählung, the census of Jewish soldiers in the German Army, in 1916 and tried to convince the newly appointed Prussian minister of war Hermann von Stein to publicly restore the honour of Jewish soldiers.

Stein, however, only assured Cassel that "the behaviour of Jewish soldiers and fellow citizens during the war gave nocause for the order by my predecessor, and thus cannot be connected with it".

[6][7] After World War I Cassel represented the German Democratic Party in the Prussian constitutional assembly.